Gallium nitride-based compound semiconductor light-emitting device

ABSTRACT

The present disclosure relates to a gallium nitride-based compound semiconductor light-emitting device with low driving voltage and high light emission output, which has a positive electrode comprising a transparent electrically conducting layer put into direct contact with a p-type semiconductor layer. An embodiment of the disclosure includes a gallium nitride-based compound semiconductor light-emitting device, which includes an n-type semiconductor layer, a light-emitting layer and a p-type semiconductor layer, which may be formed in this order on a substrate, wherein each layer comprises a gallium nitride-based compound semiconductor, the light-emitting device has a negative electrode and a positive electrode provided on the n-type semiconductor layer and a region having a semiconductor metal concentration of 20 at. % or more, based on all metals, is present in the transparent electrically conducting film on the semiconductor side surface of the transparent electrically conducting film.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Rule 53(b) Continuation of U.S. application Ser.No. 12/093,758 filed May 15, 2008, which is a 371 of PCT Application No.PCT/JP2006/323052 filed Nov. 14, 2006, and which claims benefit ofJapanese Patent Application No. 2005-331607 filed Nov. 16, 2005 and U.S.Provisional Application No. 60/739,003 filed Nov. 23, 2005. Theabove-noted applications are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a gallium nitride based compoundsemiconductor light-emitting device. More specifically, the presentinvention relates to a face-up type gallium nitride-based compoundsemiconductor light-emitting device equipped with a positive electrodehaving excellent light-transparency and ohmic properties.

BACKGROUND ART

In recent years, a GaN-based compound semiconductor material hasattracted attention as a semiconductor material for short-wavelengthlight-emitting devices.

The GaN-based compound semiconductor is formed on various oxidesubstrates such as sapphire single crystal or a Group III-V compoundsubstrate by a metal organic chemical vapor deposition method (MOCVDmethod), a molecular beam epitaxy (MBE method) or the like.

A characteristic feature of the GaN-based compound semiconductormaterial is that the current diffusion in the transverse direction issmall. This is attributable to many dislocations present in theepitaxial crystal and penetrating from the substrate to the surface, butthe details are not known. Furthermore, in a p-type GaNbased compoundsemiconductor, the resistivity is high as compared with the resistivityof an n-type GaN-based compound semiconductor and, therefore, unless thematerial coming into ohmic contact with the p-type GaN-based compoundsemiconductor is made to be a positive electrode, the driving voltagegreatly increases.

The material coming into ohmic contact with the p-type GaN-basedcompound semiconductor is mainly a metal and, in particular, a metalhaving a high work function readily establishes ohmic contact. Also,from the standpoint of light penetration, the positive electrode ispreferably transparent to light. Therefore, a metal material whicheasily makes ohmic contact with a p-type GaN-based compoundsemiconductor has heretofore been formed into a thin film to therebyachieve both low resistance and light transparency.

When a metal is formed into a thin film, there arises a problem that theresistance in the diffusion direction becomes high. To solve thisproblem, a positive electrode having a two-layer structure consisting ofan ohmic contact layer comprising a thin-film metal and a currentdiffusion layer comprising a transparent electrically conducting filmhaving high light transmittance has been proposed (see, for example,Japanese Patent No. 294173).

In order to fabricate a brighter light-emitting device (LED), thepresence of a metal layer which reflects or absorbs light must beeliminated. For this purpose, a method of bringing a transparentelectrically conducting film itself into ohmic contact with a p-typeGaN-based compound semiconductor has been studied (see, for example,Japanese Unexamined Patent Publication No. 2001-210867). In JapaneseUnexamined Patent Publication No. 2001-210867, it is proposed to producea transparent electrically conducting film coming into direct contactwith a p-type GaN-based compound semiconductor by a method other than asputtering method. In the sputtering method, the contact resistance ishigh because the p-type GaN-based compound semiconductor layer isdamaged, and a low operating voltage cannot be obtained. However, when atransparent electrically conducting film is formed by a method otherthan sputtering and, then, the thickness of the transparent electricallyconducting film is increased by a sputtering method, as the film-formingmethod is changed on the way of constructing a stacked structure, theresistance increases at the interface where the method is changed. Also,the productivity is poor.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a gallium nitride-basedcompound semiconductor light-emitting device with low driving voltageand high light emission output, which has a positive electrodecomprising a transparent electrically conducting layer put into directcontact with a p-type semiconductor layer. Another object of the presentinvention is to provide a gallium nitride-based compound semiconductorlight-emitting device having a positive electrode with low contactresistance and excellent current diffusibility, where a plurality oftransparent electrically conducting film layers having differentpurposes are formed by a consistent film-forming technique withoutdamaging a p-type semiconductor layer at the formation of transparentelectrically conducting film layers.

The present inventors have found, for example, that: when asemiconductor metal mixed layer and a positive electrode metal mixedlayer are appropriately formed on the semiconductor side surface of atransparent electrically conducting film layer constituting a positiveelectrode and on the positive electrode side surface of a p-typesemiconductor, respectively, low contact resistance can be obtained;when a transparent electrically conducting film layer is formed to havea stacked structure including at least two layers of a transparentelectrically conducting film contact layer put into direct contact witha p-type GaN-based compound semiconductor and a transparent electricallyconducting film current diffusion layer and the transparent electricallyconducting film contact layer is formed by an RF sputtering method, thetransparent electrically conducting film layer can be formed withoutdamaging a p-type gallium nitride-based compound semiconductor; when atransparent electrically conducting film current diffusion layer iscontinuously stacked on a transparent electrically conducting filmcontact layer by a DC sputtering method, the increase in resistance atthe interface between these two layers can be suppressed; when atransparent electrically conducting film current diffusion layer isformed to have a large thickness, the diffusion of current can beincreased while maintaining high light transparency; and when atransparent electrically conducting film layer is heat-treated after thefilm formation, a large concentration gradient of a semiconductor metalis generated in the semiconductor metal mixed layer. The presentinvention has been accomplished based on these findings.

That is, the present invention provides the following inventions.

(1) A gallium nitride-based compound semiconductor light-emitting devicecomprising an n-type semiconductor layer, a light-emitting layer and ap-type semiconductor layer which are formed in this order on asubstrate, wherein each layer comprises a gallium nitride-based compoundsemiconductor, the light-emitting device has a negative electrode and apositive electrode provided on the n-type semiconductor layer and on thep-type semiconductor layer, respectively, the positive electrode is atleast partially formed of a transparent electrically conducting film,the transparent electrically conducting film is at least partially incontact with the p-type semiconductor layer, a semiconductor metal mixedlayer containing a Group III metal component is present on thesemiconductor side surface of the transparent electrically conductingfilm, and the thickness of the semiconductor metal mixed layer is from0.1 to 10 nm.

(2) The gallium nitride-based compound semiconductor light-emittingdevice according to item 1, above, wherein a region having asemiconductor metal concentration of 20 at. % or more based on allmetals is present in the semiconductor metal mixed layer.

(3) The gallium nitride-based compound semiconductor light-emittingdevice according to item 1 or 2 above, wherein a region having asemiconductor metal concentration of 40 at. % or more based on allmetals is present in the range of less than 3 nm from thesemiconductor/transparent electrically conducting film interface of thesemiconductor metal mixed layer.

(4) The gallium nitride-based compound semiconductor light-emittingdevice according to any one of items 1 to 3 above, wherein thesemiconductor metal concentration in the range of 3 nm or more from thesemiconductor/transparent electrically conducting film interface of thesemiconductor metal mixed layer is 15 at. % or less based on all metals.

(5) The gallium nitride-based compound semiconductor light-emittingdevice according to any one of items 1 to 4 above, wherein thetransparent electrically conducting film is formed at room temperatureand, after the film formation, is heat-treated at 300 to 700° C.

(6) The gallium nitride-based compound semiconductor light-emittingdevice according to any one of items 1 to 5 above, wherein thetransparent electrically conducting film comprises a transparentelectrically conducting film contact layer and a transparentelectrically conducting film current diffusion layer, and thetransparent electrically conducting film contact layer is in contactwith the p-type semiconductor layer.

(7) The gallium nitride-based compound semiconductor light-emittingdevice according to item 6 above, wherein the transparent electricallyconducting film contact layer is formed by an RF sputtering method.

(8) The gallium nitride-based compound semiconductor light-emittingdevice according to item 6 or 7 above, wherein the transparentelectrically conducting film current diffusion layer is formed by a DCsputtering method.

(9) The gallium nitride-based compound semiconductor light-emittingdevice according to any one of items 6 to 8 above, wherein the thicknessof the transparent electrically conducting film contact layer is from 1to 5 nm.

(10) The gallium nitride-based compound semiconductor light-emittingdevice according to any one of items 6 to 9 above, wherein the thicknessof the transparent electrically conducting film current diffusion layeris from 150 to 500 nm.

(11) The gallium nitride-based compound semiconductor light-emittingdevice according to any one of items 6 to 10 above, wherein thetransparent electrically conducting film current diffusion layer takesdifferent structures between the sides close to and remote from thep-type semiconductor and the structure of the layer on the side remotefrom the semiconductor is a columnar structure.

(12) The gallium nitride-based compound semiconductor light-emittingdevice according to item 11 above, wherein the film thickness of thestructure layer on the side closer to the semiconductor is from 30 to100 nm.

(13) The gallium nitride-based compound semiconductor light-emittingdevice according to any one of items 1 to 12 above, wherein a positiveelectrode metal mixed layer containing a metal component of thetransparent electrically conducting film is present in the p-typesemiconductor layer.

(14) The gallium nitride-based compound semiconductor light-emittingdevice according to item 13 above, wherein the thickness of the positiveelectrode metal mixed layer is from 0.1 to 5 nm.

(15) The gallium nitride-based compound semiconductor light-emittingdevice according to item 13 or 14 above, wherein the concentration ofthe transparent electrically conducting film metal component in thepositive electrode metal mixed layer is from 0.1 to 20 at. % based onthe all metals in the positive electrode metal mixed layer.

(16) The gallium nitride-based compound semiconductor light-emittingdevice according to any one of items 1 to 15 above, wherein thetransparent electrically conducting film comprises an oxide of at leastone metal selected from the group consisting of In, Sn, Zn, Al, Ti, V,Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Sb,Hf, Ta, W, Re, Os, Ir and Pt.

(17) The gallium nitride-based compound semiconductor light-emittingdevice according to item 16 above, wherein the transparent electricallyconducting film comprises an oxide of at least one metal selected fromthe group consisting of In, Sn, Zn, Al, Cu, Ag, Ga, Ge, W, Mo and Cr.

(18) A lamp comprising the gallium nitride-based compound semiconductorlight-emitting device according to any one of items 1 to 17 above.

(19.) An electronic device incorporating the lamp according to item 18above.

(20) A machine incorporating the electronic device according to item 19above.

The gallium nitride-based compound semiconductor light-emitting deviceof the present invention appropriately has a semiconductor metal mixedlayer containing a Group III metal, constituting the semiconductor, onthe semiconductor side surface of the transparent electricallyconducting film layer constituting the positive electrode, so that thecontact resistance between the positive electrode and the p-typesemiconductor layer can be small. Also, in the semiconductor metal mixedlayer, a large concentration gradient of the semiconductor metal isgenerated to have a high semiconductor metal proportion in the vicinityof the semiconductor/transparent electrically conducting film layerinterface and a low semiconductor metal proportion at the positionremote from the interface, so that the contact resistance between thesemiconductor and the transparent electrically conducting film layer canbe reduced in the vicinity of the interface, good crystallinity of thesemiconductor can be maintained at the position remote from theinterface, and the light-emitting device can have a low resistance.

Furthermore, a positive electrode metal mixed layer containing a metalconstituting the transparent electrically conducting film layer of thepositive electrode is caused to be appropriately present on the positiveelectrode side surface of the p-type semiconductor layer, so that thecontact resistance between the semiconductor and the transparentelectrically conducting film layer can be more reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a general structure of theinventive gallium nitride-based compound semiconductor light-emittingdevice;

FIG. 2 is a schematic view illustrating an embodiment of the inventivegallium nitride-based compound semiconductor light-emitting device;

FIG. 3 is a schematic view showing the cross-section of a galliumnitride-based compound semiconductor light-emitting device produced inExample 1;

FIG. 4 is a schematic view showing the planar surface of a galliumnitride-based compound semiconductor light-emitting device produced inExample 1;

FIG. 5 is an exemplary chart showing results of the EDS analysis of asectional TEM image of a transparent electrically conducting filmcontact layer;

FIG. 6 is an exemplary chart showing results of the EDS analysis of asectional TEM image of a p-type contact layer; and

FIG. 7 is a sectional TEM photograph of the transparent electricallyconducting film current diffusion layer of a gallium nitride-basedcompound semiconductor light-emitting device produced in Example 1.

BEST MODES FOR CARRYING OUT THE INVENTION

The gallium nitride-based compound semiconductor light-emitting deviceof the present invention is a semiconductor light-emitting devicefabricated by, as shown in FIG. 1, stacking a gallium nitride-basedcompound semiconductor on a substrate (1), through a buffer layer (2) asneeded, forming an n-type semiconductor layer (3), a light-emittinglayer (4) and a p-type semiconductor layer (5), partially removing thelight-emitting layer and the p-type semiconductor layer, forming anegative electrode (20) on the exposed n-type semiconductor layer, andforming a positive electrode (10) on the remaining p-type semiconductorlayer.

For the substrate, known substrate materials can be used without anyrestrictions, examples including: an oxide single crystal such as asapphire single crystal (Al₂O₃; A-plane, C-plane, M-plane, R-plane), aspinel single crystal (MgAl₂O₄), a ZnO single crystal, a LiAlO₂ singlecrystal, LiGaO₂ single crystal, and a MgO single crystal; a Si singlecrystal; a SiC single crystal; a GaAs single crystal; an AlN singlecrystal; a GaN single crystal; and a boride single crystal such as ZrB₂.The plane orientation of the substrate is not limited to any specificdirection. The crystal plane of the substrate may be inclined to aspecific crystal plane or not inclined.

For the construction of the n-type semiconductor layer, thelight-emitting layer, and the p-type semiconductor layer, there arevarious known structures which can be used without any restrictions. Thep-type semiconductor layer may have a conventional carrierconcentration. Notably, the transparent electrode of the presentinvention may also be applicable to a p-type semiconductor layer havinga low carrier concentration (e.g., about 1×10¹⁷ cm⁻³).

For the gallium nitride-based compound semiconductor used to form theselayers, semiconductors of various compositions, represented by thegeneral formula Al_(x)In_(y)Ga_(1-x-y)N (0≦x<1, 0≦y<1, 0≦x+y<1), areknown, and any of the semiconductors of various compositions,represented by the general formula Al_(x)In_(y)Ga_(1-x-y)N (0≦x<1,0≦y<1, 0≦x+y<1), including the known ones, can be used without anyrestrictions as the gallium nitride-based compound semiconductor forforming the n-type semiconductor layer, the light-emitting layer and thep-type semiconductor layer in the present invention.

The method for growing such gallium nitride-based compoundsemiconductors is not specifically limited, and any known method forgrowing gallium nitride-based compound semiconductors, such as MOCVD(Metal Organic Chemical Vapor Deposition), HVPE (Hydride Vapor PhaseEpitaxy), and MBE (Molecular Beam Epitaxy), can be used. MOCVD is thepreferred growth method from the viewpoint of the controllability of thefilm thickness and mass-producibility. In the MOCVD method, hydrogen(H₂) or nitrogen (N₂) is used as the carrier gas, and trimethylgallium(TMG) or triethylgallium (TEG), trimethylaluminum (TMA) ortriethylaluminum (TEA), and trimethylindium (TMI) or triethylindium(TEI) are used as the Ga source, the Al source, and the In source,respectively, which are the source materials from group III, whileammonia (NH₃), hydrazine (N₂H₄), etc. are used as the N sources, thesource materials from group V. As for the dopants, mono-silane (SiH₄) ordi-silane (Si₂H₆) as the Si source material and germane (GeH₄) or anorganic germanium compound as the Ge source material are used for then-type, and bis(cyclopentadienyl)magnesium (Cp₂Mg) orbis(ethylcyclopentadienyl)magnesium ((EtCp)₂Mg), for example, is used asthe Mg source material for the p-type.

To form the negative electrode in contact with the n-type semiconductorlayer in the gallium nitride-based compound semiconductor fabricated byforming the n-type semiconductor layer, the light-emitting layer, andthe p-type semiconductor layer in this order on the substrate, thelight-emitting layer and the p-type semiconductor layer are partiallyremoved to expose the underlying n-type semiconductor layer. After that,the inventive transparent positive electrode is formed on the unremovedregion of the p-type semiconductor layer, and the negative electrode isformed on the exposed n-type semiconductor layer. Negative electrodes ofvarious compositions and structures are known, and any of negativeelectrodes of various compositions and structures, including the knownones, can be used as the negative electrode without any restrictions.

For instance, there can be used the stacked structure shown in FIG. 2that is obtained by successively stacking, on a sapphire substrate (1),a buffer layer (2) composed of an AlN, a n-type semiconductor layer (3)which consists of an n-type contact layer (3 a) composed of an n-typeGaN and an n-type clad layer (3 b) composed of an n-type GaN, alight-emitting layer (4) composed of an InGaN, and p-type semiconductorlayer (5) which consists of an p-type contact layer (5 a) composed of ap-type GaN and a p-type clad layer (5 b) composed of a p-type AlGaN.

The p-type contact layer (5 a), p-type clad layer (5 b), light-emittinglayer (4) and n-type clad layer (3 b) of the gallium nitride-basedcompound semiconductor stacked structure are partly removed by etching,and a conventional negative electrode (20) of, for example, Ti/Au isformed on the exposed n-type contact layer (3 a), and a positiveelectrode (10) is formed on the remained p-type contact layer (5 a).

In the present invention, the positive electrode (10) has at least atransparent electrically conducting film (11) put into contact with thep-type semiconductor layer. On a part of the transparent electricallyconducting film (11), a bonding pad layer (15) is provided forestablishing electrical connection with a circuit board, a lead frame orthe like.

The transparent electrically conducting film is transparent to light inthe emission wavelength region and composed of a material havingelectrical conductivity. For example, the transparent electricallyconducting film is composed of an oxide of at least one metal selectedfrom the group consisting of In, Sn, Zn, Al, Ti, V, Cr, Mn, Fe, Co, Ni,Cu, Ga, Ge, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Sb, Hf, Ta, W, Re, Os, Irand Pt. Among these, an oxide of at least one metal selected from thegroup consisting of In, Sn, Zn, Al, Cu, Ag, Ga, Ge, W, Mo and Cr ispreferred because of its good light transparency and high electricalconductivity. Particularly, ITO in various compositions is preferred.

The thickness of the transparent electrically conducting film ispreferably from 50 to 1,000 nm. If the thickness is less than 50 nm, thesheet resistance value is disadvantageously low, whereas if it exceeds1,000 nm, the productivity is worsened. The thickness is more preferablyfrom 100 to 500 nm, still more preferably from 150 to 300 nm.

The transparent electrically conducting film is preferably constructedto have a two-layer structure consisting of a transparent electricallyconducting film contact layer and a transparent electrically conductingfilm current diffusion layer. The transparent electrically conductingfilm contact layer provided in contact with the p-type semiconductorlayer is formed without damaging the p-type semiconductor layer at thefilm formation. For this purpose, the energy entering the p-typesemiconductor layer needs to be made as low as possible. The transparentelectrically conducting film contact layer is formed with weak incidentenergy and therefore, the density of the film becomes low. The thicknessof the transparent electrically conducting film contact layer ispreferably from 1 to 5 nm. If the film thickness is less than 1 nm, thep-type semiconductor layer is damaged at the formation of thetransparent electrically conducting film diffusion layer, whereas if itexceeds 5 nm, the current diffusion effect is weak in the transparentelectrically conducting film contact layer and the current diffusioneffect of the entire transparent electrically conducting film isreduced. The film thickness is more preferably from 1.5 to 3.5 nm.

The transparent electrically conducting film current diffusion layersubsequently formed satisfies both high light transmittance and lowsheet resistance. The transparent electrically conducting film currentdiffusion layer is preferably a high-density film for enhancing thecurrent diffusion effect. The thickness of the transparent electricallyconducting film current diffusion layer is preferably from 50 to 1,000nm. If the film thickness is less than 50 nm, low sheet resistancecannot be obtained, whereas if it exceeds 1,000 nm, high lighttransmittance cannot be obtained. The film thickness is more preferablyfrom 150 to 700 nm. These two layers are preferably formed continuouslywithout intermittence. If a span of time intervenes or the system istransferred to another apparatus, contamination may adhere between twolayers or the metal oxide film may be further oxidized to form ahigh-resistance layer.

When a semiconductor metal mixed layer containing a metal constitutingthe semiconductor is caused to be present on the semiconductor sidesurface of the transparent electrically conducting film, the contactresistance between the transparent electrically conducting film and thesemiconductor decreases. That is, in the present invention, the“semiconductor metal mixed layer” is defined as a semiconductormetal-containing layer in the transparent electrically conducting film,and the layer containing the semiconductor metal component in an amountof 3 at. % or more based on all metal components, which is present inthe transparent electrically conducting layer, is referred to as a“semiconductor metal mixed layer”.

The thickness of the semiconductor metal mixed layer is preferably from0.1 to 10 nm. If the thickness is less than 0.1 nm or exceeds 10 nm, lowcontact resistance can be difficult to obtain. In order to obtain lowercontact resistance, the thickness is more preferably from 1 to 8 nm.

As for the proportion of the semiconductor metal contained in thesemiconductor metal mixed layer, a region containing the semiconductormetal at a proportion of 20 at. % or more based on all metal componentsis preferably present. The proportion has a distribution, and theproportion of the semiconductor-forming metal is higher as closer to thesemiconductor/transparent electrically conducting film interface.Particularly, a region, where the semiconductor metal proportion is 40at. % or more based on all metals in the semiconductor metal mixedlayer, is preferably present in the range of less than 3 nm from thesemiconductor/transparent electrically conducting film interface. Thismeans that the diffusion of the semiconductor metal into the transparentelectrically conducting film is proceeding in the vicinity of thesemiconductor/transparent electrically conducting film interface, thatis, in the range of less than 3 nm from the interface, and thereby thecontact resistance is decreased.

The details of this mechanism have not been elucidated but, in view ofthe energy level diagram at the contact interface of p-typeGaN/transparent electrically conducting film, a hole and an electron arelocated in closer distance to each other when diffusion is proceeding ascompared with the case of no occurrence of diffusion, and a largernumber of recombination centers are thought to be generated. It may alsobe presumed that when a recombination center is present, holes orelectrons flow into that region to give an electrically neutral and lowbarrier state.

On the other hand, in the range 3 nm or more remote from thesemiconductor/transparent electrically conducting film interface, theproportion of the semiconductor metal is preferably 15 at. % or less. Ifthe semiconductor metal proportion exceeds 15 at. % in this range, thedriving voltage of the light-emitting device sometimes increases. Thatis, the transparent electrically conducting film 3 nm or more remotefrom the semiconductor/transparent electrically conducting filminterface is preferably in a state that the diffusion of thesemiconductor metal is not proceeding. This is because if the diffusionof the metal constituting the semiconductor excessively proceeds, thecrystal of the semiconductor layer is destroyed and the semiconductorlayer comes to have high resistance.

The thickness of the semiconductor metal mixed layer and proportion ofthe semiconductor-forming metal contained in the layer can be measuredby the EDS analysis of sectional TEM image, as is well known to thoseskilled in the art. Thus, in regard to the transparent electricallyconducting film, EDS analysis of a sectional TEM image can be performedat several points, for example five points, in thickness direction fromthe p-type semiconductor/transparent electrically conducting filminterface, and type and content of metal contained at each point can bedetermined from each chart at these points. If five measurement pointsare insufficient to determine the thickness, measurement can be made atseveral additional points.

Also, a positive electrode metal mixed layer, containing the metalconstituting the transparent electrically conducting film (i.e. thepositive electrode), is preferably present on the surface of the p-typesemiconductor layer on the side of the positive electrode. With suchconstruction, contact resistance between the transparent electricallyconducting film and the p-type semiconductor layer can be furtherdecreased. In short, a “positive electrode metal mixed layer”, as usedherein, is defined as a layer containing the metal constituting thetransparent electrically conducting film, in the p-type semiconductorlayer.

The electrical resistance of the positive electrode metal mixed layer ishigher than that of other p-type semiconductor layers but, as thecontact resistance between semiconductor and positive electrode becomeslow, the driving voltage can be made lowest by forming the positiveelectrode metal mixed layer in an appropriate film thickness.

The thickness of the positive electrode metal mixed layer is preferablyfrom 0.1 to 5 nm. If the thickness is less than 0.1 nm, the effect ofreducing the contact resistance is not sufficiently high, whereas if itexceeds 5 nm, the crystallinity of the semiconductor layer surface isdisadvantageously destroyed. The thickness is more preferably from 1 to3 nm.

The proportion of the transparent electrically conductingfilm-constituting metal contained in the positive electrode metal mixedlayer is preferably from 0.1 to 30 at. %. If the proportion is less than0.1%, the effect of reducing the contact resistance is not sufficientlyhigh, whereas if it exceeds 30 at. %, the crystallinity of thesemiconductor layer surface may be destroyed to increase the resistivityof the semiconductor layer. The proportion is more preferably from 1 to20 at. %.

The thickness and positive electrode metal content of the positiveelectrode metal mixed layer can be measured by the EDS analysis of asectional TEM image similarly to the semiconductor metal mixed layer.

The methods for forming the transparent electrically conducting film,semiconductor metal mixed layer and positive electrode metal mixed layerare described below.

Formation of the transparent electrically conducting film on a p-typesemiconductor layer (i.e. formation of the transparent electricallyconducting film contact layer) is preferably carried out throughsputtering based on RF discharge. It has been elucidated that anelectrode exhibiting low contact resistance can be formed through RFdischarge sputtering rather than through vapor deposition or DCdischarge sputtering.

In film formation through RF discharge sputtering, sputtered atomsdeposited on the p-type semiconductor layer gain energy throughion-assisting effect. Thus, diffusion of the sputtered atoms in thesurface portion of p-type semiconductor (e.g., Mg-doped p-type GaN) isconsidered to be promoted. In addition, atoms forming the top surface ofthe p-type semiconductor are imparted with energy during film formation.Thus, diffusion of a semiconductor material (e.g., Ga) into thetransparent electrically conducting film contact layer is considered tobe promoted. Through EDS analysis of a sectional TEM image of thetransparent electrically conducting film contact layer (i.e., filmformed on the p-type GaN layer through RF sputtering), a portioncontaining both Ga originating from the semiconductor (i.e., asemiconductor metal mixed layer) was observed (see FIG. 5 showing oneexample of analysis results of a transparent electrically conductingfilm contact layer obtained in Example 1 of the present invention).

On the other hand, a region where In and Sn derived from the transparentelectrically conducting film were detected by the EDS analysis of thesectional TEM image, that is, a positive electrode metal mixed layer wasconfirmed on the semiconductor side (see, FIG. 6 showing one example ofthe analysis results of a p-type semiconductor layer obtained in Example1 of the present invention).

In the sputtering of metal oxide, when the GaN layer surface is exposedto a plasma at the sputtering, the plasma particle may destroy thecrystallinity of the GaN surface. Evidence revealing destruction of thecrystallinity is not observed, but high proportion of the semiconductormetal in the semiconductor metal mixed layer and an increase in the filmthickness of the mixed layer result. This is considered to occur becausebecause the crystallinity of the GaN surface is first destroyed by aplasma particle and the transparent electrically conducting film is thenformed, the semiconductor metal, of which the crystal structure isdestroyed much more, diffuses into the transparent electricallyconducting film.

Accordingly, in order to prevent an increase in the contact resistance,the GaN stacked substrate must be devised not to be exposed to a plasmaat the sputtering. Examples of the method therefor include increasingthe T-S (target-substrate) distance, increasing the magnetic force ofthe magnet, and devising the magnet shape to prevent the plasma fromspreading toward the substrate direction.

Furthermore, the formation of the transparent electrically conductingfilm contact layer is preferably performed at a temperature not higherthan room temperature. If the p-type semiconductor is heated, theincident particle of the transparent electrically conducting filmcontact layer is imparted with a diffusion energy from the p-typesemiconductor and excessively diffuses to destroy the crystallinity ofthe semiconductor and form a high-resistance layer.

Heating at 300 to 700° C. after the formation of the transparentelectrically conducting film produces a state where diffusion of thetransparent electrically conducting film-forming metal and thesemiconductor-forming metal is proceeding in the vicinity of thesemiconductor/transparent electrically conducting film interface, thatis, in the range of less than 3 nm from the interface. At this heattreatment, a long-time treatment allowing for diffusion into the deeppart of the semiconductor layer is not performed. The treatment time ispreferably from 1 to 30 minutes.

In the case of forming a transparent electrically conducting filmcurrent diffusion layer subsequently to the transparent electricallyconducting film contact layer, the current diffusion layer is formed bya sputtering method using DC discharge. The current diffusion layerformed by DC discharge sputtering is advantageous in that the density ofthe transparent electrically conducing film becomes higher than that inthe film formation by RF discharge sputtering.

Also, when the transparent electrically conducting layer is formed atroom temperature by a DC discharge sputtering method, the currentdiffusion layer takes different structures between the sides close toand remote from the p-type semiconductor, and the layer on the sideremote from the semiconductor side of the current diffusion layer comesto have a more distinct columnar structure with a high density. Thelayer close to the semiconductor side forms a structure which is not acolumnar structure, though a domain is observed. FIG. 7 shows asectional TEM photograph of the transparent electrically conducting filmcurrent diffusion layer in Example 1 of the present invention. As seenfrom the Figure, a distinct columnar structure is observed in theportion of A. In the portion of B, a domain is observed but thestructure is not a columnar structure. The film thickness of thisportion is usually from 30 to 100 nm and, by forming the currentdiffusion layer to a film thickness larger than that, the portion of Ahaving a columnar structure with high crystallinity can be created. C isthe semiconductor layer. Incidentally, presence of the transparentelectrically conducting film contact layer cannot be confirmed at thismagnification.

Sputtering may be carried out using any known conventional sputteringapparatus under any suitably selected conditions conventionally known. Asubstrate having gallium nitride-based compound semiconductor layersstacked thereon is placed in the chamber. The chamber is evacuated tothe degree of vacuum in the range of 10⁻⁴˜10⁻⁷ Pa. He, Ne, Ar, Kr, Xe,etc. can be used as the sputtering gas. Ar is preferred in view ofavailability. One of these gases is introduced into the chamber up tothe pressure of 0.1˜10 Pa, and then, discharge is performed. Preferablythe pressure is in the range of 0.2˜5 Pa. Supplied electric power ispreferably in the range of 0.2˜2.0 kW. By suitably adjusting thedischarge time and supplied power, the thickness of the formed layer canbe adjusted.

As the bonding pad layer, various structures using materials such as Au,Al, Ni and Cu are well known, and these well known materials andstructures can be used with no restriction. Preferably, the thickness isin the range of 100˜1000 nm. The thickness is more preferably 300 nm ormore since higher bondability is obtained with thick bonding pad owingto the property of bonding pads. However, from the viewpoint ofproduction cost, the thickness is preferably 500 nm or less.

The gallium nitride-based compound semiconductor light-emitting deviceaccording to the present invention can be used to constitute a lamp byintegrally adding a transparent cover by means that are well known inthis art. A white lamp can also be produced by combining the galliumnitride-based compound semiconductor light-emitting device of thepresent invention with a cover containing a phosphor.

Further, a lamp fabricated from a gallium nitride-based compoundsemiconductor light-emitting device of the present invention exhibits alow driving voltage and a high emission intensity. Therefore, electronicdevices such as mobile phones and display panels, each employing a lampfabricated on the basis of the technique; and machines and apparatusessuch as automobiles, computers, and game machines, each employing any ofthe electronic device can be driven with little electric power andrealize excellent characteristics. Particularly, an electric powersaving effect is remarkably attained in mobile phones, game machines,toys, and automotive parts, which are driven by a battery.

EXAMPLES

The present invention will next be described in more detail by way ofexamples, which should not be construed as limiting the invention.

Example 1

FIG. 3 is a schematic view showing the cross-section of a galliumnitride-based compound semiconductor light-emitting device produced inthis Example, and FIG. 4 is a schematic view showing the planar surfacethereof. An n-type semiconductor layer (3) consisting of a 3 μm-thickunderlying layer (3 c) composed of undoped GaN, a 2 μm-thick n-typecontact layer (3 a) composed of Si-doped n-type GaN and a 0.03 μm-thickn-type clad layer (3 b) composed of n-type In_(0.1)Ga_(0.9)N, alight-emitting layer (4) having a multi-quantum well structure in whicha 0.03 μm-thick barrier layer composed of Si-doped GaN and a 2.5nm-thick well layer composed of In_(0.2)Ga_(0.8)N were stacked fivetimes and the barrier layer was finally provided, and a p-typesemiconductor layer (5) consisting of a 0.05 μm-thick p-type clad layer(5 b) composed of Mg-doped p-type Al_(0.07)Ga_(0.93)N and a 0.15μm-thick p-type contact layer (5 a) composed of Mg-doped p-type GaN weresequentially stacked on a sapphire substrate (1) through a buffer layer(2) composed of AlN. On the p-type contact layer of the resultinggallium nitride-based compound semiconductor stacked structure, atransparent electrically conducting film (11) consisting of a 2 nm-thicktransparent electrically conducting film contact layer (12) composed ofITO (indium tin oxide) and a 400 nm-thick transparent electricallyconducting film current diffusion layer (13) composed of ITO was formed,and a bonding pad layer (15) having a Cr/Ti/Au three-layer structure(thickness: 4/10/200 nm; Cr is on the ITO side) was formed thereon toprovide a positive electrode (10). Subsequently, a negative electrode(20) having a two-layer structure of Ti/Au was formed on the n-typecontact layer. In this way, a light-emitting device of the presentinvention, in which the semiconductor side serves as the lightextraction surface, was produced. The positive and negative electrodeseach had a shape as shown in FIG. 4.

In the above structure, the carrier concentration in the n-type contactlayer composed of n-type GaN was 1×10¹⁹ cm⁻³, the concentration of Sidoping in the barrier layer composed of GaN was 1×10¹⁸ cm⁻³, the carrierconcentration in the p-type contact layer composed of p-type AlGaN was5×10¹⁸ cm⁻³, and the concentration of Mg doping in the p-type clad layercomposed of p-type AlGaN was 5×10¹⁹ cm⁻³.

The layers forming the gallium nitride-based compound semiconductorlayer were formed by MOCVD under normal conditions well known in therelated technical field. The positive and negative electrodes werefabricated in accordance with the following procedure.

First, the portion of the n-type GaN contact layer on which the negativeelectrode was to be formed was exposed by reactive ion etching in thefollowing manner and an etching mask was formed on the p-typesemiconductor layer. The sequence of processing is as follows. Afterapplying a resist uniformly over the entire surface, the resist wasremoved from the positive electrode area by using a known lithographictechnique. Then, the structure was placed in a vacuum evaporationchamber, and using an electron beam method, Ni and Ti were deposited tothicknesses of about 50 nm and 300 nm, respectively, under a pressure of4×10⁻⁴ Pa or lower. After that, using a lift-off technique, the metallayers and the resist were removed from all the areas except thepositive electrode area.

Next, the substrate with the semiconductor layer fabricated thereon wasplaced on an electrode in an etching chamber of a reactive ion etchingapparatus, and the etching chamber was evacuated to 10⁻⁴ Pa, after whichCl₂ as an etching gas was supplied and etching was performed until then-type contact layer was exposed. After the etching, the substrate wasretrieved from the reactive ion etching apparatus, and the etching maskwas removed using nitric acid and hydrofluoric acid.

Thereafter, a transparent electrically conducting film contact layercomposed of ITO and a transparent electrically conducting film currentdiffusion layer composed of ITO were formed by known photolithographytechnique and lift-off technique on the p-type contact layer only in theregion where the positive electrode is formed. In the formation of thetransparent electrically conducting film contact layer and thetransparent electrically conducting film current diffusion layer, thesubstrate having stacked thereon the gallium nitride-based compoundsemiconductor layer was placed in a sputtering apparatus and in thestate of the substrate temperature being room temperature, ITO of about2 nm was first film-formed by RF sputtering on the p-type contact layerand ITO of about 400 nm was then stacked thereon by DC sputtering.Incidentally, at the film formation by RF sputtering, the pressure wasabout 1.0 Pa, and the supply power was 0.5 kW, and at the film formationby DC sputtering, the pressure was about 0.8 Pa, and the supply powerwas 0.5 kW. The substrate was taken out from the sputtering apparatusand treated according to the known procedure usually called lift-off.Subsequently, a first layer composed of Cr, a second layer composed ofTi and a third layer composed of Au were sequentially stacked by a vapordeposition method on a part of the transparent electrically conductingfilm current diffusion layer to form a bonding pad layer. In this way, apositive electrode was formed on the p-type contact layer.

The positive electrode formed by this method exhibited lighttransparency and had a light transmittance of 90% or more in thewavelength region of 460 nm. Incidentally, the light transmittance wasmeasured with a spectrophotometer by using a sample for measurement oflight transmittance prepared by stacking a transparent electricallyconducting film contact layer and a transparent electrically conductingfilm current diffusion layer, each having the same thickness as above,on a glass plate. The value of light transmittance was calculated bytaking into consideration the blank in the measurement only with a glassplate.

The wafer after the film formation up to the positive electrode washeat-treated in an oven at 550° C. for 10 minutes.

Next, the negative electrode was formed on the exposed n-type contactlayer in accordance with the following procedure. After applying aresist uniformly over the entire surface, the resist was removed fromthe negative electrode area on the exposed n-type contact layer by usinga known lithographic technique; then, using the conventional vacuumevaporation method, the negative electrode was formed by depositing Tiand Au to thicknesses of 100 nm and 200 nm, respectively, in this orderon the semiconductor. After that, the resist was removed using a knowntechnique.

After the substrate thickness was reduced to 80 μm by grinding andpolishing the back surface of the substrate, the wafer with the positiveand negative electrodes formed thereon was scribed from thesemiconductor layer side by using a laser scriber and, then, stressedand broken apart, thereby separating individual chips each 350 μmsquare. When each chip was measured by flowing a current of 20 mA usinga needle-tipped probe, the forward voltage was 3.3 V.

Then, the chip was mounted in a TO-18 can package, and when the lightemission output was measured by a tester, the output at the appliedcurrent of 20 mA was 10 mW. Distribution of emission from thelight-emitting surface indicated that light emission occurred in theentire area of the light-emitting surface corresponding to the positiveelectrode.

Through EDS analysis of the sectional TEM images, the thickness of thesemiconductor metal mixed layer was estimated to be 3 nm, and the Gaproportion in the layer was estimated to 3 to 50 at. % with respect toall metal atoms (In+Sn+Ga). At the portion 1 nm remote from thesemiconductor/positive electrode interface the Ga proportion was 50 at.% with respect to all metal atoms and at the portion 3 nm remote fromthe semiconductor/positive electrode interface the Ga proportion was 10at. % or less with respect to all metal atoms. On the other hand, thethickness of the positive electrode metal mixed layer was estimated tobe 2 nm. The positive electrode materials existing in the layer were Inand Sn constituting the transparent electrically conducting film. Thesemetal proportion in the layer was estimated to 1 to 10 at. % withrespect to all metal atoms (In+Sn+Ga). FIG. 5 is an exemplary chartshowing results of the EDS analysis of a sectional TEM image of atransparent electrically conducting film contact layer, and FIG. 6 is anexemplary chart showing results of the EDS analysis of a sectional TEMimage of a p-type contact layer.

Examples 2 and 3

Gallium nitride-based compound semiconductor light-emitting devices wereproduced in the same manner as in Example 1 except for changing the filmformation condition of the positive electrode (the pressure of thesputtering apparatus at the formation of the transparent electricallyconducting film contact layer), and the device characteristics wereevaluated. The results obtained are shown in Table 1 together with theresults of Example 1. Incidentally, in these light-emitting devices, thethickness of the positive electrode metal mixed layer was from 1 to 5nm, and the proportion of the positive electrode metal was from 0.5 to18%. As for the semiconductor metal mixed layer, these are shown inTable 1.

TABLE 1 Pressure at Formation of Semiconductor Metal Mixed LayerTransparent Proportion of Semiconductor Metal [at. %] DeviceCharacteristics Electrically Portion of 1 nm from Portion of 3 nm fromForward Light conducting Film Thickness p-Type Contact Layer p-TypeContact Layer Voltage Emission Contact Layer [Pa] [nm] InterfaceInterface [V] Output [mW] Example 1 1 3 50 10 3.3 9 Example 2 0.3 3.5 5020 3.6 9 Example 3 5 3 35 8 3.5 9 Example 4 1 3 35 10 3.5 9 Example 5 14 55 20 3.8 9 Comparative 1 12 60 30 4.2 10 Example 1

Example 4

A gallium nitride-based compound semiconductor light-emitting device wasproduced in the same manner as in Example 1 except for not performingthe heat treatment after the formation of the positive electrode. Itsdevice characteristics are shown also in Table 1.

Example 5

A gallium nitride-based compound semiconductor light-emitting device wasproduced in the same manner as in Example 1 except for heating thesubstrate to 200° C. at the formation of the transparent electricallyconducting film contact layer. Its device characteristics are shown alsoin Table 1.

Comparative Example 1

A gallium nitride-based compound semiconductor light-emitting device wasproduced in the same manner as in Example 1 except for not forming thetransparent electrically conducting film contact layer. Its devicecharacteristics are shown also in Table 1.

INDUSTRIAL APPLICABILITY

The gallium nitride-based compound semiconductor light-emitting deviceprovided by the present invention has excellent characteristics andproductivity and is useful as a material for light-emitting diodes,lamps and the like.

1. A gallium nitride-based compound semiconductor light-emitting devicecomprising an n-type semiconductor layer, a light-emitting layer and ap-type semiconductor layer which are formed in this order on asubstrate, wherein each layer comprises a gallium nitride-based compoundsemiconductor, the light-emitting device has a negative electrode and apositive electrode provided on the n-type semiconductor layer and on thep-type semiconductor layer, respectively, the positive electrode is atleast partially formed of a transparent electrically conducting filmcomprising a transparent electrically conducting material, thetransparent electrically conducting film is at least partially incontact with the p-type semiconductor layer, and a region having asemiconductor metal concentration of 20 at. % or more based on allmetals is present in the transparent electrically conducting film on thesemiconductor side surface of the transparent electrically conductingfilm.
 2. The gallium nitride-based compound semiconductor light-emittingdevice according to claim 1, wherein a region having a semiconductormetal concentration of 40 at. % or more based on all metals is presentin the transparent electrically conducting film in the range of lessthan 3 nm from the semiconductor/transparent electrically conductingfilm interface.
 3. The gallium nitride-based compound semiconductorlight-emitting device according to claim 1, wherein the semiconductormetal concentration of the transparent electrically conducting film inthe range of less than 3 nm or more from the semiconductor/transparentelectrically conducting film interface is 15 at. % or less based on allmetals.
 4. The gallium nitride-based compound semiconductorlight-emitting device according to claim 1, wherein the transparentelectrically conducting film is formed at room temperature and, afterthe film formation, is heat-treated at 300 to 700° C.
 5. The galliumnitride-based compound semiconductor light-emitting device according toclaim 1, wherein the transparent electrically conducting film comprisesa transparent electrically conducting film contact layer and atransparent electrically conducting film current diffusion layer, andthe transparent electrically conducting film contact layer is in contactwith the p-type semiconductor layer.
 6. The gallium nitride-basedcompound semiconductor light-emitting device according to claim 5,wherein the transparent electrically conducting film contact layer isformed by an RF sputtering method.
 7. The gallium nitride-based compoundsemiconductor light-emitting device according to claim 5, wherein thetransparent electrically conducting film current diffusion layer isformed by a DC sputtering method.
 8. The gallium nitride-based compoundsemiconductor light-emitting device according to claim 5, wherein thethickness of the transparent electrically conducting film layer is from1 to 5 nm.
 9. The gallium nitride-based compound semiconductorlight-emitting device according to claim 5, wherein the thickness of thetransparent electrically conducting film contact layer is from 150 to500 nm.
 10. The gallium nitride-based compound semiconductorlight-emitting device according to claim 5, wherein the transparentelectrically conducting film current diffusion layer has a columnarstructure in a side remote from the p-type semiconductor-layer, and astructure different from a columnar structure in a side close to thep-type semiconductor layer.
 11. The gallium nitride-based compoundsemiconductor light-emitting device according to claim 10, wherein thefilm thickness of the structure layer on the side closer to thesemiconductor is from 30 to 100 nm.
 12. The gallium nitride-basedcompound semiconductor light-emitting device according to claim 1,wherein a positive electrode metal mixed layer containing a metalcomponent of the transparent electrically conducting film is present inthe p-type semiconductor layer.
 13. The gallium nitride-based compoundsemiconductor light-emitting device according to claim 12, wherein thethickness of the positive electrode metal mixed layer is from 0.1 to 5nm.
 14. The gallium nitride-based compound semiconductor light-emittingdevice according to claim 12, wherein the concentration of thetransparent electrically conducting film metal component in the positiveelectrode metal mixed layer is from 0.1 to 20 at. % based on the allmetals in the positive electrode metal mixed layer.
 15. The galliumnitride-based compound semiconductor light-emitting device according toclaim 1, wherein the transparent electrically conducting film comprisesan oxide of at least one metal selected from the group consisting of In,Sn, Zn, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Y, Zr, Nb, Mo, Tc,Ru, Rh, Pd, Ag, Sb, Hf, Ta, W, Re, Os, Ir and Pt.
 16. The galliumnitride-based compound semiconductor light-emitting device according toclaim 15, wherein the transparent electrically conducting film comprisesan oxide of at least one metal selected from the group consisting of In,Sn, Zn, Al, Cu, Ag, Ga, Ge, Mo, W, and Cr.
 17. A lamp comprising thegallium nitride-based compound semiconductor light-emitting deviceaccording to claim
 1. 18. An electronic device incorporating the lampaccording to claim
 17. 19. A machine incorporating the electronic deviceaccording to claim 18.